Blood Alcohol (BAC) Calculator (Widmark estimate)

Understanding the blood alcohol (BAC) calculator — concept and principle

This calculator gives a rough estimate of blood alcohol concentration (BAC) with the Widmark formula, from sex, body weight, the drinks consumed and the time since the first drink. It also shows the peak value, grams of pure alcohol, standard drinks and units, and the hours until BAC falls below common legal limits. It is meant for education about how alcohol accumulates and clears, not for deciding whether to drive.

Widmark: alcohol ÷ body water — with the default inputs: 0.058%.

How it is calculated

Pure alcohol (g) = drinks × volume (mL) × ABV × 0.789 g/mL
Peak BAC (%) = alcohol (g) ÷ (body weight (g) × r) × 100, with r = 0.68 for men and 0.55 for women
BAC now = peak − 0.015 × hours since the first drink
Hours until a level = (BAC now − level) ÷ 0.015
US standard drinks = g ÷ 14; UK units = g ÷ 8

The principle

Erik Widmark showed that alcohol spreads through the body's water, so the same amount gives a higher concentration in a smaller body or one with less water. The factor r represents that share of body weight; women on average have less body water per kilogram. The liver then removes alcohol at a roughly constant rate, typically about 0.015% per hour, though real rates range from about 0.010 to 0.025. Food, medication, health and drinking speed all change the real curve.

Worked example

Defaults: man, 70 kg, 3 drinks of 355 mL at 5%, 2 hours since the first drink.

Alcohol = 3 × 355 × 0.05 × 0.789 = 42 g = 3 US standard drinks = 5.3 UK units.
Peak = 42.01 ÷ (70,000 × 0.68) × 100 = 0.088%.
Now = 0.088 − 0.015 × 2 = 0.058% = 0.58 ‰.
Hours until about 0.00% = 0.058 ÷ 0.015 = 3.9; below 0.03% = 1.9; below 0.05% = 0.6; below 0.08% = 0.

Each point is the calculator re-run with only “Body weight (kg)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Body weight (kg)	Estimated BAC now (%)
17.5	0.323
35	0.147
52.5	0.088
70	0.058
87.5	0.041
105	0.029
140	0.014

How to read the result

Where does your result fall?

The estimate is for a population average and can be wrong by a wide margin for any individual. An estimate below the legal limit does not mean you are safe to drive: impairment starts at low levels, and limits differ by country, for example 0.03% in Korea and 0.08% in the US and England and Wales. The only safe choice is not to drive after drinking.

Common mistakes

Using the result to decide whether to drive.
Assuming coffee, a shower or sleep speeds up elimination.
Forgetting that drinks with food absorb more slowly but still count.
Underestimating pour sizes and ABV.

Keep in mind: Never use this estimate to decide whether you can drive or operate machinery. It is not medical or legal advice.

Frequently asked questions

How long does it take for alcohol to leave the body?

On average BAC falls about 0.015% per hour, but rates vary between people.

What is the legal limit in Korea?

0.03% BAC for driving under the Road Traffic Act.

Why do women reach higher BAC?

On average they have a smaller share of body water, so the same alcohol is more concentrated.

Sources: Washington State Patrol breath-test training material, "Widmark's Equation" (2002) · NIAAA, standard drink of 14 g; NHS, UK unit of 8 g · Korea Road Traffic Act, Article 44

Checked 9 Nov 2026. Widmark equation with r = 0.68 (men) / 0.55 (women) and β = 0.015 g/100 mL per hour, as used in Washington State Patrol breath-test training material (“Widmark’s Equation”, 2002); ethanol density 0.789 g/mL; US standard drink 14 g (NIAAA); UK unit 8 g (NHS); legal limits: Korea 0.03% (Road Traffic Act art. 44), US 0.08%, England & Wales 0.08%.

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